Process for the production of a monolayer composite article, the monolayer composite article and a ballistic-resistant article

ABSTRACT

Process for the production of a monolayer composite article comprising an unidirectional array of high performance polyolefin fibers, the process comprising the steps of positioning of the fibers in a coplanar, parallel fashion consolidation of the fibers to obtain the monolayer composite article, the process comprises after the step of position of the fibers and before or after the step of consolidation of the fibers, a step in which the fibers are stretched.

The invention relates to a process for the production of a monolayercomposite article comprising a unidirectional array of high performancepolyolefin fibers, the process comprising the steps of

positioning of the fibers in a coplanar, parallel fashion

consolidation of the fibers to obtain the monolayer composite article.

The invention also relates to the monolayer composite article and to aballistic-resistant article comprising the monolayer composite article.Ballistic resistant articles may be used in, for instance, helmets, asinserts in bulletproof vests, as armouring on military vehicles and inballistic-resistant panels.

A ballistic-resistant article of this type is disclosed in EP-A-833742.The known ballistic-resistant article affords already good protectionagainst impacts of projectiles such as shrapnel or bullets. The level ofprotection may be quantified by means of the Energy Absorption (Eabs) orby means of the Specific Energy Absorption (SEA), a measure of theamount of energy that can be absorbed by an article on impact of aprojectile per unit aerial density of the article.

Although the known ballistic-resistant article affords already goodprotection against impacts of projectiles there is still a great needfor ballistic-resistant articles that can offer increased protectionagainst impacts of projectiles of various kinds, especially againstprojectiles in the form of bullets.

Object of the invention is therefore to provide a process for theproduction of a monolayer composite article, that when applied in aballistic-resistant article provides improved protection.

Surprisingly this object is obtained by providing a process forproducing the monolayer composite article, which process comprises afterthe step of positioning of the fibers in a coplanar, parallel fashionand before or after the step of consolidation of the fibers in themonolayer composite article, a step in which the fibers are stretched.

Ballistic-resistant articles comprising the composite produced by theprocess according to the invention show a remarkable improvedprotection. Such a level of protection cannot be obtained by simplystretching the fibers any further in the production process of thefibers. One of the reasons is that by further drawing the fibers in theproduction process, frequent break of the fibers occurs, which isdetrimental for a smooth production process of fibers with a high andconstant quality level.

Owing to the surprisingly high level of protection achieved, not onlyarticles with an even further increased level of protection for a givenweight of the article are now available but also articles affording thesame level of protection as the known article at a significantly lowerweight. Low weight per unit area is of great importance in manyapplications. This is the case, for instance, in the field of personalprotective equipment such as helmets, shields, shoes and the like. Lowweight is also essential for the application of ballistic-resistantarticles in for instance helicopters, motorcars and high-speed, highlymaneuverable combat vehicles.

In the context of the present application monolayer composite articlemeans a layer of substantially coplanar, parallel fibers, beingconsolidated so that they maintain their coplanar, parallel fashion.Preferably a plastic is used for the consolidation, for example byembedding the fibers partially or wholly in the plastic in this wayserving as matrix material and binding the fibers together. Suchmonolayer composite articles and methods of obtaining such a monolayercomposite article are disclosed in for instance EP-B-0.191.306 and WO95/00318. A monolayer composite article may be obtained by for instancepulling a number of fibers from fiber bobbins located on a fiber bobbinframe over a comb so that they are oriented in coplanar and parallelfashion in one plane and then consolidating the fibers in that coplanarand parallel fashion, for example by embedding the fibers in the plasticmatrix material.

The stretching of the fibers may take place by increasing the transportvelocity of the fibers at a position in the process line for theproduction of the monolayer composite article. Preferably this isaccomplished by transporting the fibers over at least a first and atleast a subsequent second transportation roll, the second transportationroll having a tangential velocity at its surface that is higher than thetangential velocity at its surface of the first roll. The velocity ofthe fibers is equal to the tangential velocity of the transportationrolls at their surfaces, which equals the product of angular velocity ofthe rolls and their radii.

In order to reduce slip of the fibers at the surface of the rolls thecontact surface of the fibers with the rolls are preferably large. Mostpreferably two sets of rolls are used having the same tangentialvelocity in a set, in the first set comprising the first transportationroll, the second set comprising the second transportation roll.

Very good results are obtained if the tangential velocity of the secondroll is at most 3 times the tangential velocity of the first roll. Morepreferably the tangential velocity of the second roll is at most 2times, most preferably at most 1.5 times the tangential velocity of thefirst roll. Preferably the tangential velocity of the second roll is atleast 1.05 times, more preferably at least 1.10 times, even morepreferably at least 1.15, most preferably at least 1.25 times thetangential velocity of the first roll.

The fibers may be stretched at any temperature, as long as thetemperature is not that high that the fibers loose their mechanicalproperties. Therefore the fibers are preferably stretched at atemperature below 160° C. In the event that the fibers are based on highmolecular weight polyethylene the fibers are preferably stretched at atemperature below 155° C. In order to decrease the forces that need tobe applied to stretch the fibers, stretching is carried out at elevatedtemperature, for instance between 60 and 160° C. Preferably the fibersare stretched at a temperature above 140° C., more preferably above 145°C.

Good results are obtained if immediately after stretching the fibers arequenched to a lower temperature, preferably below 100° C., morepreferably below 80° C., most preferably below 60° C. Quenching isfavourably carried out by cooling the fibers at the secondtransportation roll, or at a further roll immediately after the secondtransportation roll. It is also possible to cool the fibers by sprayingthe fibers with a water-based emulsion of the plastic matrix material.The fibers may be kept at the stretching temperature for about 10seconds to about 5 minutes. In such case stretching starts duringheating up the fibers to the stretching temperature, or as soon aspossible if the fibers have reached that temperature. Heating the fibersmay simply be accomplished by transporting the fibers through an oven,which oven is positioned in the production line between the first andthe second transportation rolls. Preferably the fibers are kept undertension during heating up and cooling down, before and after thestretching step.

In the process for producing the monolayer composite article accordingto the invention, fibers may be used that have previously been coatedwith a polymer other than the plastic matrix material in order to, forinstance, protect the fibers during handling or in order to obtainbetter adhesion of the fibers onto the plastic matrix material.

Consolidation by embedding the fibers in the plastic matrix material maybe effected by applying one or more films of the plastic to the top, thebottom or to both sides of the plane of the fibers and then passingthese, together with the fibers, through a set of heated pressure rolls.Preferably, however, the fibers are consolidated by coating the fiberswith an amount of a liquid substance containing the plastic matrixmaterial. The advantage of this is that more rapid and betterimpregnation of the fibers is achieved. The liquid substance may be forexample a solution, a dispersion or a melt of the plastic. If a solutionor a dispersion of the plastic is used in the manufacture of themonolayer composite article, the process also comprises evaporating thesolvent or dispersant.

Further methods of consolidation may comprise sticking a plastic film atone or both surfaces of the layer of fibers, sticking plastic tapes atone or at both surfaces of the layer of fibers. In this case the fibersare only embedded for a small part.

The step of stretching the fibers may be before or after theconsolidation of the fibers. Preferably the fibers are stretched afterconsolidation. In that case it is possible to stretch to very highstretch ratios and still having a smooth running continuous process. Incase of stretching before the consolidation of the fibers, good resultsare obtained if the fibers are stretched, kept under tension after thestretching step, while consolidating the fibers by applying the plasticmatrix material. During the step of stretching the fibers, preferably atleast 10, more preferably at least 25, even more preferably at least 50and even more preferably at least 75 fibers are stretchedsimultaneously.

High performance polyolefin fibers are known to the skilled person. Thefibers have an elongate body whose length dimension is greater than thetransverse dimensions of width and thickness. The term “fibers” includesa monofilament, a multifilament yarn, a tape, a strip, a thread, astaple fiber yarn and other elongate objects having a regular orirregular cross-section. For application of the fibers inballistic-resistant articles it is essential that the fibers beballistically effective, which, more specifically, requires that theyhave a high tensile strength, a high tensile modulus and/or high energyabsorption. It is preferred for the fibers to have a tensile strength ofat least 1.2 GPa and a tensile modulus of at least 40 GPa.

Homopolymers and copolymers of polyethylene and polypropylene areparticularly suitable as polyolefins for the production of the highperformance polyolefin fibers. Furthermore, the polyolefins used maycontain small amounts of one or more other polymers, in particular otheralkene-1-polymers.

It is preferred for the reinforcing fibers in the monolayer compositearticle to be of high-molecular weight linear polyethylene, having aweight average molecular weight of at least 400,000 g/mol, morepreferably having a weight average molecular weight of at least 800,000g/mol, even more preferably having a weight average molecular weight ofat least 1,200,000 g/mol. Most preferably the reinforcing fibers ofhigh-molecular weight linear polyethylene have a weight averagemolecular weight of at least 2,500,000 g/mol.

Linear polyethylene here means polyethylene having less than 1 sidechain per 100 C atoms, preferably less than 1 side chain per 300 Catoms.

Preferably, use is made of polyethylene fibers consisting ofpolyethylene filaments prepared by a gel spinning process as describedin for example GB-A-2042414 and GB-A-2051667. This process essentiallycomprises the preparation of a solution of a polyolefin of highintrinsic viscosity, spinning the solution to filaments at a temperatureabove the dissolving temperature, cooling down the filaments below thegelling temperature so that gelling occurs and drawing the filamentsbefore, during or after removal of the solvent.

The shape of the cross-section of the filaments may be selected herethrough selection of the shape of the spinning aperture.

Preferably, use is made of multifilament yarns of ultrahigh molecularweight linear polyethylene with an intrinsic viscosity of at least 5dl/g, determined in decalin at 135° C., and a yarn titre of at least 50denier, with the yarn having a tensile strength of at least 25, morepreferably at least 30, even more preferably at least 32, even morepreferably at least 34 cN/dtex and a tensile modulus of at least 1000cN/dtex. Preferably the filaments have a cross-section aspect ratio ofat most 3. The use of these fibers has been found to improve the highlevel of protection of the ballistic-resistant article of the inventionstill further.

The plastic matrix material may wholly or partially consist of a polymermaterial, and optionally may contain fillers usually employed forpolymers. The polymer may be a thermoset or thermoplastic or mixtures ofboth. In one preferred embodiment a soft plastic is used, in particularit is preferred for the plastic matrix material to be an elastomer witha tensile modulus (at 25° C.) of at most 41 MPa. Preferably, theelongation to break of the plastic is greater than the elongation tobreak of the reinforcing fibers. The elongation to break of the matrixpreferably is from 3 to 500%.

Thermosets and thermoplastics that are suitable for the monolayercomposite article are listed in for instance WO-A-91/12136 (line 26,page 15 to line 23, page 21). Preferably, vinylesters, unsaturatedpolyesters, epoxies or phenol resins are chosen as matrix material fromthe group of thermosetting polymers. These thermosets usually are in themonolayer in partially set condition (the so-called B stage) before thestack of monolayers is cured during compression of theballistic-resistant article. From the group of thermoplastic polymerspolyurethanes, polyvinyls, polyacryls, polyolefins or thermoplastic,elastomeric block copolymers such aspolyisoprene-polyethylene-butylene-polystyrene orpolystyrene-polyisoprene-polystyrene block copolymers are preferablychosen as matrix material.

The plastic matrix material content of the monolayer composite articleis chosen sufficiently low, for example to save weight, preferably lowerthan 30 wt. % relative to the total weight of the monolayer. Morepreferably, the content is lower than 20 wt. %, most preferably lowerthan 10 wt. %.

The invention also relates to a monolayer composite article obtainableby the process according to the invention as outlined above.

The invention also relates to an article comprising polyolefin highperformance fibers having a stretch ratio at break of less than 1.4,preferably less than 1.35, more preferably less than 1.30, morepreferably less than 1.25 more preferably less than 1.20, morepreferably less than 1.15, most preferably less than 1.1, whereby thestretch ratio at break is measured at a temperature of 150° C. and at adeformation rate of 0.2 min⁻¹.

Preferably such an article is a monolayer composite article as definedabove. More preferably such an article comprises a monolayer compositearticle as defined above.

A further preferred article is a cross-layered composite article,comprising at least one pair of the monolayer composite articlesaccording to the invention, the fiber direction in each monolayercomposite article in the cross-layered article of the invention isrotated with respect to the fiber direction in an adjacent monolayercomposite article. Good results are achieved when this rotation amountsto at least 45 degrees. Preferably, this rotation amounts toapproximately 90 degrees.

Further preferred articles include a ballistic-resistant article for useas protective means. It is known how to produce such ballistic-resistantarticles comprising a monolayer composite article.

Normally in a first step a stack comprising several monolayer compositearticles is produced. Preferably the fiber direction in each monolayercomposite article in the ballistic-resistant article of the invention isrotated with respect to the fiber direction in an adjacent monolayer.Preferably the stack is made out of the cross-layered composite articlesaccording to the invention. Preferably the monolayer composite articlesin the cross-layered composite article are interconnected e.g. throughcalendaring. Calendaring conditions such as temperature and pressure arechosen sufficiently high to prevent delamination of the monolayercomposite articles, while on the other hand not too high to preventdeterioration of fiber properties e.g. due to melting of the fiber.Typical ranges for temperature are preferably between 75 and 155° C., atypical pressure will be preferably at least 0.05 MPa. The deteriorationof the fiber properties subsequently are reflected in a reducedanti-ballistic performance. Good conditions for temperature and pressurecan be found by the skilled man with some routing experimentation withinthe above mentioned boundaries.

In a further step the stack may be enclosed in an envelope or connectedby sewing. In this way a flexible ballistic-resistant article isobtained, for instance for use in a bullet resistant or bulletproofvest, that is suitable for use under normal clothing.

Ballistic-resistant articles with a very high level of protection areobtained if elevated temperature and pressure are applied to the stack,so that the monolayer composite articles or the cross-layered compositearticle are adhered by moulding. These articles are rigidballistic-resistant articles. Good examples are helmets, shields, armourpanels for use in vehicles and aircraft, inserts in for example bulletresistant vests etc.

The present invention leads to composite articles and ballisticresistant articles showing improved protection compared to the knownarticles. Therefore in one aspect the invention also relates to amonolayer composite articles and to a cross-layered composite articlethat show a v₅₀ of at least 380 m/s, if produced into flexible compositearticle, comprising a stack of the monolayer composite article or thecross-layered composite article, the flexible composite article havingan aerial density between 1.95 and 2.05 kg/m² and shot by a 9 mmparabellum having a weight of 8 gram, according to STANAG 2920.Preferably the v₅₀ is at least 400 m/s, more preferably at least 420m/s, more preferably at least 450 m/s, more preferably at least 480 m/s,more preferably at least 520 m/s, more preferably at least 560 m/s, mostpreferably at least 600 m/s. It will be clear that the areal density maybe increased by the use of a larger amount of monolayer compositearticles and/or cross-layered composite article if higher v₅₀ values arerequired.

In another aspect the invention relates to a flexible ballisticresistant article, preferably a bullet resistant vest, having a SEA, ifshot by a 9 mm parabellum having a weight of 8 gram, according to STANAG2920, of at least 300 J.m²/kg, more preferably at least 350 J.m²/kg,more preferably at least 400 J.m²/kg, most preferably at least 450J.m²/kg.

In yet another aspect the invention relates to a rigidballistic-resistant article, this article having an aerial densitybetween 1.9 and 2.1 kg/m² and having a v₅₀, if shot by a 9 mm parabellumhaving a weight of 8 gram, according to STANAG 2920 of at least 400 m/s,more preferably at least 420 m/s, more preferably at least 450 m/s, morepreferably at least 480 m/s, more preferably at least 520 m/s, morepreferably at least 560 m/s, most preferably at least 600 m/s. It willbe clear that the areal density may be increased by the use of a largeramount of monolayer composite articles and/or cross-layered compositearticle if higher v₅₀ values are required.

The invention will be further explained in the examples.

Measurements

Determination of stretch ratio at break of fibers in monolayer compositearticle. A sample having a width of 10 mm and a length of 1 metercomprising coplanar fibers of the same length is taken out of themonolayer composite article according to the invention. The sample isplaced in a universal tensile testing machine in an oven at 150° C.,under a small tension of about 3% of the breaking load, in order toavoid the fibers to shrink. Once the temperature equilibrium in the ovenis established the sample is drawn with a deformation rate of 0.2 min⁻¹until rupture of the sample. The stretch ratio at break is the length atbreak of the sample/original length of sample that is stretched.

The value is taken as the average of 5 measurements.

It is also possible to obtain a single fiber out of an article and tomeasure the ratio of break at the same temperature and deformation rate.

Production of Flexible or Rigid Ballistic-Resistant Articles

A stack of cross-layered composite articles is made. The angle betweenthe fiber directions in subsequent monolayers in the stack is always 90degrees. The stacks have an aerial density of 2.0 kg/m²+/−0.1 kg/m², anddimensions of 0.4 m*0.4 m. In case of a flexible ballistic-resistantarticle the stack of cross-layered composite articles is fixed aroundthe perimeter by sewing. In case of a rigid ballistic resistant articlethe stack is consolidated in a heating press at 120° C. and 75 bars for30 minutes.

Determination of v₅₀, and Specific Energy Absorption (SEA)

The v₅₀ Of the ballistic resistant articles is determined by using 9 mmParrabellum bullets according to Stanag 2920. The SEA is calculatedaccording to the formula:

SEA=0.5*m*v₅₀ ²/AD,

in which formulaSEA is specific energy absorption (J. m²/kg).m is the mass of the bullet (8 gram).v₅₀ is the velocity in m/s of the bullets at which 50% of the bulletsare stopped by the ballistic-resistant article.AD is the aerial density of the articles (kg/m²).

Determination of Intrinsic Viscosity, IV

The Intrinsic Viscosity is determined according to method PTC-179(Hercules nc. Rev. Apr. 29, 1982) at 135° C. in decalin, the dissolutiontime being 16 hours, with DBPC as anti-oxidant in an amount of 2 g/lsolution, by extrapolating the viscosity as measured at differentconcentrations to zero concentration;

Comparative Experiment A

A monolayer composite article was produced out of Dyneema® SK 76, 1760dtex fibers were used. A ply of 4 monolayers in a cross-layers fashionhaving at both sides an Ildpe cover film were produced. A ply has anaerial density of 145 g/m², of which 72% was due to the fibers, 18% wasdue to matrix material, which is a SIS rubber, and 10% was due to theIldpe films.

Flexible ballistic composite articles were produced, by making a stackof 14 plies and sewing the stack around the perimeter. The articles weretested according to Stanag 2920. Also the stretch ratio at break of thefibers was determined. Results are presented in table 1

Comparative Experiment B

A monolayer composite article was produced out of Dyneema® SK 76, 1760dtex fibers were used. A ply of 4 monolayers in a cross-layered fashionwas produced. A ply has an aerial density of 260 g/m², of which 80% wasdue to the fibers and 20% was due to the matrix material, which is a SISrubber. Rigid ballistic-resistant articles were produced by making astack of 8 plies and by compressing the stack as indicated above.

EXAMPLE 1

Comparative experiment A was repeated, however during the production ofthe monolayer composite article, before the application of the matrixmaterial, the fibers in the monolayer composite article were stretchedwith a stretch ratio of 1.33. The aerial density of the ply was 113g/m2, and the ballistic-resistant article comprised 18 plies. Resultsare presented in table 1.

EXAMPLE 2

Comparative experiment A was repeated, however during the production ofthe monolayer composite article, before the application of the matrixmaterial, the fibers in the monolayer composite article were stretchedwith a stretch ratio of 1.44. The aerial density of the monolayercomposite article was 105 g/m2, and the ballistic-resistant articlecomprised 19 plies. Results are presented in table 1.

EXAMPLE 3

Comparative experiment B was repeated, however during the production ofthe monolayer composite article, before the application of the matrixmaterial, the fibers in the monolayer composite article were stretchedwith a stretch ratio of 1.33. The aerial density of the ply was 195g/m2, and the ballistic-resistant article comprised 10 plies. Resultsare presented in table 1.

EXAMPLE 4

Comparative experiment B was repeated, however during the production ofthe monolayer composite article, before the application of the matrixmaterial, the fibers in the monolayer composite article were stretchedwith a stretch ratio of 1.44. The aerial density of the ply was 180g/m2, and the ballistic-resistant article comprised 11 plies. Resultsare presented in table 1.

Comparative Stretch ratio Stretch ratio at SEA exp./Example (—) break.V₅₀ (m/s) (J/(kg/m2)) A 1 1.58 375 280 B 1 1.63 355 251 1 1.33 1.28 425365 2 1.44 1.19 449 403 3 1.33 1.29 461 422 4 1.44 1.20 477 455

It is clear from the results in table 1 that considerable improvedvalues for the v₅₀ and the SEA are obtained, which values are higherthan those ever obtained before.

1. Process for the production of a monolayer composite articlecomprising an unidirectional array of high performance polyolefinfibers, the process comprising the steps of positioning of the fibers ina coplanar, parallel fashion consolidation of the fibers to obtain themonolayer composite article, characterized in that, the processcomprises after the step of position of the fibers and before or afterthe step of consolidation of the fibers, a step in which the fibers arestretched.
 2. Process for the production of a monolayer compositearticle according to claim 1, wherein a plastic matrix material is usedfor the consolidation.
 3. Process for the production of a monolayercomposite article according to claim 2, wherein the fibers areconsolidated by embedding the fibers partially or wholly in a plasticmatrix material.
 4. Process for the production of a monolayer compositearticle according to claim 1, wherein the stretching of the fibers takesplace by increasing the transport velocity of the fibers at a positionin the process line for the production of the monolayer compositearticle.
 5. Process for the production of a monolayer composite articleaccording to claim 4, wherein the increase in the transport velocity isaccomplished by transporting the fibers over at least a first and atleast a subsequent second transportation roll, the second transportationroll having a tangential velocity at its surface that is higher than thetangential velocity at its surface of the first roll.
 6. Process for theproduction of a monolayer composite article according to claim 4,wherein the increase in transport velocity is at most a factor of
 3. 7.Process for the production of a monolayer composite article according toclaim 4, wherein the increase in transport velocity is at least a factorof 1.05.
 8. Process for the production of a monolayer composite articleaccording to claim 1, wherein the step of stretching the fibers is afterthe consolidation of the fibers.
 9. Process for the production of across-layered composite article whereby at least one pair of monolayercomposite articles according to claim 1 is stacked whereby the fiberdirection in each monolayer composite article is rotated with respect tothe fiber direction in an adjacent monolayer.
 10. Monolayer compositearticle obtainable by the process according to claim
 1. 11. Articlecomprising a high performance polyolefin fiber having a stretch ratio atbreak of less than 1.4, measured at 150° C. and with a deformation rateof 0.2 min⁻¹.
 12. Article according to claim 11, which article comprisesa high performance polyolefin fiber having a stretch ratio at break ofless than 1.35, measured at 150° C. and with a deformation rate of 0.2min⁻¹.
 13. A monolayer composite articles or a cross-layered compositearticle that show a V₅₀ of at least 380 m/s, if produced into flexiblecomposite article, comprising a stack of the monolayer composite articleor the cross-layered composite article, the flexible composite articlehaving an aerial density between 1.95 and 2.05 kg/m² and shot by a 9 mmparabellum having a weight of 8 gram, according to STANAG
 2920. 14. Aflexible ballistic resistant article, preferably a bullet resistantvest, having a SEA, if shot by a 9 mm parabellum having a weight of 8gram, according to STANAG 2929, of at least 300 J.m²/kg.
 15. A rigidballistic resistant article, preferably a bullet resistant vest, havinga SEA, if shot by a 9 mm parabellum having a weight of 8 gram, accordingto STANAG 2929, of at least 300 J.m²/kg.
 16. Process according to claim1 or an article, wherein the high performance polyolefin fibers have astrength of at least 1.2 GPa and a modulus of at least 40 GPa. 17.Process according to claim 1 or an article, wherein the high performancepolyolefin fibers are obtained by the gel spinning process.
 18. Processor article according to claim 1, wherein the high performance polyolefinfibers are fibers of high molecular weight linear polyethylene having aweight average molecular weight of at least 400,000 g/mol.